Here is the original item:
Professor Neal loves chocolate cake.
(A) But could
not accept the slice that her student Michael
offered (B)
because she knew that it was a
bribe (C)
for a better grade on his midterm essay.
You wanted to do this:
Professor Neal loves chocolate cake. But could not accept the slice that
her student Michael offered. Because she knew
that it was a bribe for a better grade on his midterm essay.
A period after offered makes a fragment. Because she knew ... is a subordinate clause, which cannot stand alone as a complete sentence.
You might want to review the rules.